In the Fables universe, the blanket term "Fable" is used, but really any character from fables, folklore, fairy tales, myths, legends, or any number of literary sources (like Alice's Adventures in Wonderland, for instance) is pretty much fair game. So within the universe you may see anyone from Cinderella or Lancelot to Pinocchio, Peter Piper, or Reynard the Fox.

As far as who I'd like to see? Well, I think I've mentioned this in some other thread, but since I've read the books and I know some of the characters I'd love to see Rose Red, Boy Blue, Flycatcher, Frau Totenkinder, and Jack make appearances. If I'm not thinking of the comics and I'm thinking of other characters that would be interesting to see, Hans Christian Andersen's Little Mermaid comes to mind (sure, she may have been turned into sea foam in the story, but Bluebeard was also stabbed to death by his brothers-in-law and he managed to get better. )

It would also be nice to see characters that aren't from the series. I have always wanted Peter Pan in Fables. He was actually suppose to be the "Adversary" in the comic series, but the creator ran into problems with that.

It would be cool to see Charming, Rose Red, Totekinder, and Old King Cole. They are staples in the comics. I myself am biased towards the great American folk tales. So seeing someone like Paul Bunyan, Pecos Bill, or especially John Henry would be most cool.

I remember Neil Gaiman's American Gods made fun of Paul Bunyan, since the bulk of the 'legend' surrounding him was made up for an advertising campaign in the 1920s rather than being genuine folklore. It would be interesting to see how a Fable would cope with having the bulk of his widely known qualities stem not from traditional folklore, but from an advertising gimmick.